Implement a SubstringGenerator that generates all substrings of a string. For example, the substrings of the string "rum" are the seven strings
"r", "ru", "rum", "u", "um", "m", ""
Hint: First enumerate all substrings that start with the first character. There are n of them if the string has length n. Then enumerate the substrings of the string that you obtain by removing the first character.